1. Field of the Invention
The present invention relates to a batch CVD (chemical vapor deposition) method and apparatus, and particularly to a semiconductor processing technique for forming a product film on target objects, such as semiconductor wafers. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target object, such as a semiconductor wafer or a glass substrate used for an FPD (Flat Panel Display), e.g., an LCD (Liquid Crystal Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target object.
2. Description of the Related Art
In manufacturing semiconductor devices for constituting semiconductor integrated circuits, a target object, such as a semiconductor wafer, is subjected to various processes, such as film formation, etching, oxidation, diffusion, and reformation. Film formation processes of this kind are performed in film formation apparatuses of the single-substrate type, such as an apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 09-077593, and film formation apparatuses of the batch type, such as an apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-006801.
FIG. 5 is a structural view schematically showing a conventional batch CVD apparatus. For example, where a silicon oxide film is formed, a wafer boat 4 holding target objects or semiconductor wafers W at intervals in the vertical direction is loaded into a vertical process container 2. The wafers W are heated to a predetermined temperature, such as about 600° C., by a heater 6 disposed around the process container 2. A silicon source gas and a reactive gas, such as ozone, are supplied from a gas supply system 8. These gases are delivered into the process container 2 from a number of gas spouting holes 8A and 10A formed on distribution nozzles 8 and 10 vertically extending inside the process container 2. Further, the inner space of the process container 2 is vacuum-exhausted by a vacuum exhaust system 14 including a vacuum pump 16 through an exhaust port 12 formed at the lower side of the process container 2. Under these conditions, a process for forming a silicon oxide film is performed inside the process container 2 with a predetermined pressure kept therein.
In recent years, owing to the demands of increased miniaturization and integration of semiconductor integrated circuits, it is required to alleviate the thermal history of semiconductor devices in manufacturing steps, thereby improving the characteristics of the devices. For vertical processing apparatuses, it is also required to improve semiconductor processing methods in accordance with the demands described above. For example, there is a CVD method for a film formation process, which performs film formation while intermittently supplying a source gas and so forth to repeatedly form layers each having an atomic or molecular level thickness, one by one, or several by several. In general, this film formation process is called ALD (Atomic layer Deposition) or MLD (Molecular Layer Deposition), which allows a predetermined process to be performed without exposing wafers to a very high temperature.
Where a silicon oxide film is formed by ALD or MLD using the apparatus shown in FIG. 5, operations are performed as follows. Specifically, a switching valve 8B for a silicon source gas and a switching valve 10B for ozone serving as an oxidizing gas are operated to alternately supply the silicon source gas and oxidizing gas. Further, the exhaust valve 14B of the vacuum exhaust system 14 is operated to adjust its valve opening degree to control the pressure inside process container 2.
FIG. 6 is a graph showing the relationship between the valve states and the pressure inside the process container where a silicon oxide film is formed by ALD using the apparatus shown in FIG. 5. FIG. 6, (A), shows the state of the source gas switching valve 8B, FIG. 6, (B), shows the state of the reactive gas switching valve 10B, FIG. 6, (C), shows the state (valve opening degree) of the exhaust valve 14B of the vacuum exhaust system, and FIG. 6, (D), shows the pressure inside the process container.
According to the method shown in FIG. 6, a cycle comprising an adsorption step T11, an exhaust step T12, a reaction step T13, and an exhaust step T14 in this order is repeated a plurality of times. In the adsorption step T11, as shown in FIG. 6, (A), the source gas switching valve 8B is set open to supply the silicon source gas, so that this gas is adsorbed on the surface of the wafers W. In the reaction step T13, as shown in FIG. 6, (B), the reactive gas switching valve 10B is set open to supply the reactive gas or ozone, so that the ozone reacts with the source gas adsorbed on the surface of the wafers, thereby forming a thin SiO2 film. In the exhaust steps T12 and T14, the exhaust valve 14B is set open to exhaust gas from inside the process container 2, without supplying either of the source gas and reactive gas.
By performing one cycle, a thin film having an atomic or molecular level thickness is formed. Thin films formed by respective times in repetition of the cycle are laminated so that a product film having a predetermined thickness is formed. In one cycle, the time length of each of the adsorption step T11 and reaction step T13 is about 60 seconds, and the time length of each of the exhaust steps T12 and T14 is about 10 seconds. This batch CVD method allows the process to be performed without exposing wafers to a very high temperature. However, as described later, the present inventors have found that batch CVD methods of this kind have room for improvement in terms of some characteristics thereof concerning the film quality, throughput, and source gas consumption.